This invention relates generally to condition responsive sensors and more particularly to fluid pressure responsive capacitive transducers.
A known fluid pressure responsive capacitive transducer, as shown in U.S. Pat. No. 4,982,351, assigned to the assignee of the present invention, the subject matter of which is incorporated herein by this reference, comprises a capacitive pressure sensing element having a thin ceramic diaphragm mounted in closely spaced, sealed overlying relation to a ceramic base and having metal coatings deposited on respective opposing surfaces of the diaphragm and the base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Sensing element terminals connected to the capacitor plates are arranged at an opposite surface of the sensing element base and a signal conditioning electrical circuit connected to the sensing element terminals is mounted on the sensing element.
A connector body of electrically insulating material is fitted over the electrical circuit and is secured to a metal housing having a cavity in which the sensing element is received. The metal housing is formed with a port for connection to a fluid pressure source to be monitored and a flexible O-ring is seated in the metal housing around the port with the sensing element biased against the O-ring to form a fluid seal and to define a fluid receiving recess with the diaphragm exposed to fluid in the recess. In that arrangement, the diaphragm is movable in response to variations in pressure applied to the diaphragm to vary the capacitance of the capacitor in accordance with the changes in applied pressure and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
As shown in FIG. 1, capacitive transducers are made in which the metal housing is in the form of a threaded port member having a polygonal, such as hexagonal, outer surface configuration to facilitate mounting of the transducer. In order to provide DC isolation, it is known to electrically couple the port member to the power supply through selected capacitors 14c (FIG. 3) by providing an electrical trace on a tab extending from the electronic circuit and received between the connector and that portion of the port member which is crimped onto the connector thereby making effective electrical connection with the electrical trace on the tab. Higher frequency AC passes from the port member to the power supply causing the sidewalls of the port member which define the cavity in which the sensing element is received to act as a shield against electromagnetic interference (EMI).
It is also known to coat the fluid receiving surface of the diaphragm with gold or the like and to electrically couple that to the electronic circuit by taking a gasket formed of electrically conductive material and placing it between the gold plated sensing surface and the port member, which, as noted above, is electrically connected to the electronic circuit. This connection provides for fluid shift correction required for sensing certain fluids, e.g., conductive fluids.
However, certain types of electrical noise can be conducted to the conditioning circuit by means of an AC signal and cause an unpredictable voltage output. For example, in certain heavy truck applications, communication between sensor units is accomplished through a power line carrier at about 130 K Hz. AC voltage potentials between circuit ground and the port member, at frequencies below 1 M Hz, cause output voltage fluctuations at multiples of the clock frequency of the ASIC of the conditioning circuitry, typically around 80 KHZ.
It is an object of the present invention to provide a fluid pressure sensor free of the above noted prior art limitations. Another object is the provision of a fluid pressure sensing capacitive transducer in which EMI (electromagnetic interference) shielding is provided as well as a conductive path for fluid shift gold plating in the transducer""s capacitive sensing element and both AC and DC isolation of the conditioning circuit.
Briefly, in accordance with the invention, the capacitive sensing element of the transducer is received in an electrically conductive cup member having a bottom wall formed with a fluid receiving aperture therethrough and an upstanding wall having a free distal end which is crimped onto the end of the connector of the transducer. The cup member is placed within a sleeve of electrically insulating material and the sleeved cup is then placed in the cavity of a port member and connected thereto by crimping the outer free end of the wall of the port member forming the cavity through the insulating sleeve member. Pressure transducers made in accordance with the invention provide EMI shielding by means of the electrically conductive cup capacitively coupled to the conditioning circuit as well as both AC and DC isolation of the conditioning circuit from the port member. If desired, fluid shift correction can be provided by means of a conductive path from a conductive coating disposed on the outer face of the diaphragm of the capacitive sensing element through a conductive gasket disposed between the diaphragm and the electrically conductive cup.